This invention relates to the control of reciprocating devices, such as pumps. It is often desirable to use a plurality of reciprocating pumps in such a way that their respective rates of fluid delivery are easily and reliably controllable, both individually and relative to each other. For example, in chemical processes it is often desired to delivery several fluid ingredients in relatively adjustable proportions. As another example, in hospitals, chemicals are frequently used in water solutions. Such solutions are not infrequently of very low concentration, e.g. 2% of the chemical to 98% water. Because it has been difficult heretofore to achieve on-the-spot mixing of such solutions in the required proportions, it has been the practice to purchase and store them ready mixed. Even in the minimum quantities needed for a reliable emergency supply, such solutions, by virtue of their low concentrations, required large storage volumes. This would have been greatly reduced if it had been practical to store only the pure chemical in liquid form and combine it only as consumed in the desired proportions with water, which is available at anytime in essentially unlimited quantities from the water supply.
Prior attempts to provide controllable pumps suitable for such purposes have been largely unsuccessful, because they have been based on technical approaches which were too complicated, too costly, and in some cases even too unreliable for the particular application.